Method of processing workpiece

ABSTRACT

A method of grinding a workpiece is carried out by a grinding apparatus including a spindle for rotating a grinding wheel, a servomotor for moving the spindle, and a servo driver for sending signals to the servomotor. The method includes a depth-of-cut command issuing step of issuing a command representing a projected depth of cut to the servo driver and a grinding step of grinding the workpiece. In the grinding step, the servo driver finishes movement of the spindle when the spindle has moved by a target distance corresponding to the projected depth of cut.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method of processing a workpiece such as a semiconductor wafer by grinding the workpiece to thin the same.

Description of the Related Art

Device chips for use in electronic equipment and appliances are formed by dividing circular semiconductor wafers, for example. Specifically, a plurality of intersecting projected dicing lines are established on a face side of a semiconductor wafer to demarcate areas on the face side where devices such as integrated circuits (ICs) are formed. Then, the semiconductor wafer is divided along the projected dicing lines into individual device chips. In recent years, there has been a significant tendency to make smaller electronic equipment and appliances, with resultant growing demands for smaller and thinner device chips for use in such smaller electronic equipment and appliances. For manufacturing thinner device chips, therefore, semiconductor wafers to be divided into device chips are thinned to a predetermined thickness by grinding.

A grinding apparatus includes a chuck table for holding a workpiece and a grinding unit disposed above the chuck table. The grinding unit includes a spindle extending generally in vertical directions, a spindle motor connected to the upper end of the spindle, and a grinding wheel connected to the lower end of the spindle. The grinding wheel includes a plurality of grinding stones arranged in an annular array on a lower surface thereof that faces an upper surface of the chuck table. The grinding apparatus includes a grinding feed unit that has a servomotor for relatively moving the spindle and the chuck table toward and away from each other and a servo driver for sending signals to the servomotor.

For grinding a workpiece on the grinding apparatus, the workpiece is fixedly placed on the chuck table, the chuck table is rotated about its central axis along directions perpendicular to the upper surface of the chuck table, and the grinding wheel is also rotated about its central axis along directions perpendicular to the lower surface thereof. Then, the spindle and the chuck table are relatively moved toward each other. The grinding stones on the grinding wheel as they move along an annular track are brought into contact with the workpiece, grinding the workpiece (see, for example, JP 2003-209080A). The grinding apparatus further includes a measuring unit for measuring the thickness of the workpiece being ground, the measuring unit being disposed in the vicinity of the chuck table. For grinding the workpiece, a control unit of the grinding apparatus issues a command to the servo driver to start moving the spindle and the chuck table relatively to each other and controls the grinding unit to grind the workpiece while the measuring unit is monitoring the thickness of the workpiece. When the workpiece that is being ground has reached a predetermined target thickness, the control unit issues a command to the servo driver to finish the relative movement of the spindle and the chuck table.

SUMMARY OF THE INVENTION

The control unit of the grinding apparatus controls operation of all units and measuring instruments of the grinding apparatus and processes various pieces of information constantly. Consequently, after the grinding unit has started grinding a workpiece, the control unit may undergo a malfunction due to an excessive processing load thereon, for example. The malfunctioning control unit is unable to issue a command to the servo driver to finish the relative movement of the spindle and the chuck table when the workpiece is thinned to the predetermined target thickness. If the relative movement of the spindle and the chuck table is not finished when it should be finished, the workpiece may not only be thinned beyond the target thickness, but also may be ground away, allowing the grinding stones to hit the chuck table and possibly resulting in serious damage to the grinding apparatus.

It is therefore an object of the present invention to provide a method of processing a workpiece on a grinding apparatus to prevent the workpiece from being excessively processed, i.e., ground, and to prevent the grinding apparatus from being damaged even in the event of a malfunction of a control unit of the grinding apparatus while the workpiece is being ground.

In accordance with an aspect of the present invention, there is provided a method of processing a workpiece by grinding the workpiece on a grinding apparatus that includes a chuck table having a holding surface for holding the workpiece thereon, the chuck table being rotatable about an axis transverse to the holding surface, a spindle for circumferentially rotating a disk-shaped grinding wheel mounted thereon and including grinding stones, a grinding feed unit having a servomotor for relatively moving the chuck table and the spindle toward and away from each other, a servo driver for sending signals to the servomotor of the grinding feed unit, and a control unit for controlling the servo driver. The method includes a holding step of holding the workpiece on the holding surface of the chuck table, a depth-of-cut command issuing step of calculating a projected depth of cut by subtracting a target thickness registered in advance in the control unit for the workpiece from a thickness of the workpiece and issuing a command representing the calculated projected depth of cut from the control unit to the servo driver of the grinding feed unit, and a grinding step of relatively moving the chuck table and the spindle toward each other with the servomotor of the grinding feed unit while the chuck table and the grinding wheel are being rotated about respective axes thereof, thereby grinding the workpiece held on the chuck table with the grinding stones. In the grinding step, relative movement of the chuck table and the spindle is finished by the servo driver when the chuck table and the spindle have relatively moved by a target distance corresponding to the projected depth of cut.

Preferably, the grinding apparatus further includes a thickness measuring unit for measuring the thickness of the workpiece held on the holding surface of the chuck table. The depth-of-cut command issuing step includes repeatedly measuring the thickness of the workpiece with the thickness measuring unit until the thickness of the workpiece reaches the target thickness, updating the projected depth of cut with a value calculated by subtracting the target thickness from the measured thickness of the workpiece, and repeatedly issuing a command representing the updated projected depth of cut from the control unit to the servo driver of the grinding feed unit. The grinding step includes updating the target distance on the basis of the updated projected depth of cut by the servo driver each time the servo driver is supplied with the command representing the updated projected depth of cut from the control unit until the chuck table and the spindle have relatively moved by the target distance.

In the method of processing a workpiece according to the aspect of the present invention, the control unit does not issue a command for starting and stopping operation to the servo driver. When the workpiece is to be ground, the control unit calculates a projected depth of cut by subtracting the target thickness for the workpiece from the thickness of the workpiece and issues a command representing the projected depth of cut to the servo driver. The servo driver finishes relative movement of the chuck table and the spindle when the chuck table and the spindle have moved by a target distance corresponding to the projected depth of cut. In this case, even if the control unit malfunctions while the workpiece is being ground, the servo driver keeps the command representing the projected depth of cut remaining therein. The servo driver finishes relative movement of the chuck table and the spindle when it has moved the chuck table and the spindle by the target distance corresponding to the projected depth of cut. Therefore, even in the event of a malfunction of the control unit while the workpiece is being ground, the workpiece is prevented from being excessively ground and the grinding apparatus is prevented from being damaged.

Therefore, in the method of processing a workpiece according to the present invention, even if the control unit of the grinding apparatus malfunctions while the workpiece is being ground, the workpiece is not likely to be excessively ground and no damage is inflicted on the grinding apparatus.

The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a grinding apparatus used in a method of processing a workpiece according to an embodiment of the present invention;

FIG. 2 is a schematic perspective view illustrating a depth-of-cut command issuing step and a grinding step of the method of processing a workpiece according to the embodiment;

FIG. 3 is a schematic cross-sectional view illustrating the depth-of-cut command issuing step and the grinding step of the method;

FIG. 4A is a flowchart of a sequence of steps of the method of processing a workpiece according to the embodiment;

FIG. 4B is a flowchart of a sequence of steps of the depth-of-cut command issuing step of the method of processing a workpiece according to the embodiment; and

FIG. 4C is a flowchart of a sequence of steps of the grinding step of the method of processing a workpiece according to the embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A method of processing a workpiece according to an embodiment of the present invention will be described below with reference to the drawings. In the method of processing a workpiece according to the embodiment, a workpiece such as a semiconductor wafer that is substantially in the shape of a circular plate is thinned to a predetermined thickness by grinding. First, the workpiece will be described below. FIG. 1 schematically illustrates in perspective the workpiece, denoted by 1. The workpiece 1 is a wafer made of silicon (Si), silicon carbide (SiC), gallium nitride (GaN), gallium arsenide (GaAs), or any of other semiconductor materials. Alternatively, the workpiece 1 may be a substrate that is substantially in the shape of a circular plate made of sapphire, glass, quartz, or the like. The glass may be alkali glass, non-alkali glass, soda-lime glass, lead glass, borosilicate glass, quartz glass, or the like. The workpiece 1 has a plurality of devices such as ICs or large-scale-integration (LSI) circuits formed on a face side 1 a thereof. When the workpiece 1 is cut along projected dicing lines between the devices, the workpiece 1 is divided into individual device chips including the respective devices. If the workpiece 1 is thinned by being ground before the workpiece 1 is divided, then thin device chips are formed from the workpiece 1 by subsequently dividing the workpiece 1. The face side 1 a of the workpiece 1 to be ground may be free of devices.

The method of processing a workpiece according to the embodiment is carried out by a grinding apparatus including a grinding unit. The grinding apparatus, denoted by 2 in FIG. 1, will be described below with reference to FIG. 1. FIG. 1 schematically illustrates the grinding apparatus 2 in perspective. The grinding apparatus 2 grinds a reverse side 1 b of the workpiece 1 which is exposed upwardly. In a case in which devices have been formed on the face side 1 a of the workpiece 1, a tape-shaped protective member 3 is affixed to the face side 1 a of the workpiece 1 in order to protect the face side 1 a. Though the workpiece 1 with the protective member 3 affixed to the reverse side 1 b will be described below by way of example, the present invention is not limited to the illustrated workpiece 1. The protective member 3 includes a base layer and a glue layer formed on the base layer. For grinding the reverse side 1 b of the workpiece 1, the face side 1 a of the workpiece 1 is supported on a chuck table 8 to be described later. If the face side 1 a and a holding surface 8 a of the chuck table 8 are held in direct contact with each other, the devices on the face side 1 a may possibly be damaged when the workpiece 1 is ground. Therefore, the protective member 3 is affixed to the face side 1 a of the workpiece 1.

The workpiece 1 is housed in a cassette 28 a that is capable of housing a plurality of workpieces 1, for example, and carried and introduced into the grinding apparatus 2. The grinding apparatus 2 includes a base 4 supporting its components thereon. Cassette rest tables 26 a and 26 b are fixed to the front end of the base 4. The cassette 28 a that houses workpieces 1 to be ground therein is placed on the cassette rest table 26 a, whereas a cassette 28 b that houses ground workpieces 1 therein is placed on the cassette rest table 2 6b. A transport robot 30 is installed on the base 4 at a position adjacent to the cassette rest tables 26 a and 26 b. The transport robot 30 unloads a workpiece 1 to be ground from the cassette 28 a on the cassette rest table 26 a and transports the unloaded workpiece 1 to a positioning table 32 on the base 4 at a position adjacent to the transport robot 30.

The positioning table 32 has a plurality of positioning pins arranged in an annular array. When a workpiece 1 is placed on a central rest area of the positioning table 32, the positioning pins are moved in unison radially inwardly to engage and position the workpiece 1 in a predetermined position on the positioning table 32. A loading arm 34 and an unloading arm 36 are disposed in a position on an upper surface of the base 4 adjacent to the positioning table 32. The workpiece 1 positioned in the predetermined position on the positioning table 32 by the positioning pins of the positioning table 32 is transported by the loading arm 34.

On a central upper surface of the base 4, there is disposed a turntable 6 that is in the form of a circular plate and is rotatable in a horizontal plane. The turntable 6 supports on its upper surface three chuck tables 8 that are angularly spaced 120 degrees from each other in circumferential directions, for example. When the turntable 6 is rotated about its central axis, the chuck tables 8 are angularly moved about the central axis of the turntable 6.

Each of the chuck tables 8 has a suction channel, not illustrated, defined therein that has an end connected to a suction source, not illustrated, and another end connected to the holding surface 8 a of the chuck table 8. FIG. 3 schematically illustrates the chuck table 8 in cross section. The holding surface 8 a is provided as an upper surface of a porous member 8 b, for example. When an object such as a workpiece 1 is placed on the holding surface 8 a, the suction source is actuated to produce a negative pressure that acts through the suction channel and the porous member 8 b on the workpiece 1, holding the workpiece 1 under suction on the holding surface 8 a. The chuck table 8 is fixedly mounted on a table support 8 d disposed on the upper surface of the turntable 6. The table support 8 d has a lower surface connected through a spindle to a table rotating motor 8 e fixedly mounted on the turntable 6. When the table rotating motor 8 e is energized, the chuck table 8 fixed to the table support 8 d is rotated about a table rotation axis 8 c that intersects with the holding surface 8 a.

Workpieces 1 can be loaded onto and unloaded from the chuck table 8 in a loading/unloading area 6 a over the turntable 6 illustrated in FIG. 1. In the loading/unloading area 6 a, the loading arm 34 can load a workpiece 1 onto one of the chuck tables 8 and the unloading arm 36 can unload a workpiece 1 from one of the chuck tables 8. For grinding a workpiece 1 on the grinding apparatus 2, the workpiece 1 is loaded onto the chuck table 8 positioned in the loading/unloading area 6 a by the loading arm 34, after which the turntable 6 is turned to move the chuck table 8 with the workpiece 1 loaded thereon to a next rough grinding area 6 b.

The grinding apparatus 2 includes a first grinding unit 10 a mounted on a rear upper surface of the base 4 outside of the turntable 6. The first grinding unit 10 a performs rough grinding on the reverse side 1 b of the workpiece 1 held on the chuck table 8 positioned in the rough grinding area 6 b. After the workpiece 1 on the chuck table 8 in the rough grinding area 6 b has been roughly ground, the turntable 6 is turned to move the chuck table 8 from the rough grinding area 6 b to a finish grinding area 6 c adjacent to the rough grinding area 6 b. The grinding apparatus 2 includes a second grinding unit 10 b mounted on another rear upper surface of the base 4 outside of the turntable 6. The second grinding unit 10 b performs finish grinding on the reverse side 1 b of the workpiece 1 held on the chuck table 8 positioned in the finish grinding area 6 c. After the workpiece 1 on the chuck table 8 in the finish grinding area 6 c has been finishingly ground, the turntable 6 is turned to move the chuck table 8 from the finish grinding area 6 c back to the loading/unloading area 6 a adjacent to the finish grinding area 6 c. The workpiece 1 that has been roughly and finishingly ground is unloaded from the chuck table 8 by the unloading arm 36.

A spinner cleaning apparatus 38 for cleaning and drying a ground workpiece 1 is disposed in the vicinity of the unloading arm 36 and the transport robot 30 on the upper surface of the base 4. The workpiece 1 that has been cleaned and dried by the spinner cleaning apparatus 38 is transported from the spinner cleaning apparatus 38 by the transport robot 30 and housed into the cassette 28 b on the cassette rest table 26 b.

The grinding apparatus 2 will further be described below with reference to FIGS. 1 and 3. A column 22 a is erected on a rear portion of the base 4. The column 22 a supports on a front face thereof a grinding feed unit 24 a that supports the first grinding unit 10 a for movement in vertical directions. The grinding feed unit 24 a includes a spindle 50 extending in the vertical directions, a servomotor 48 connected to the upper end of the spindle 50, and a servo driver 46 electrically connected to the servomotor 48. The servo driver 46 sends signals to the servomotor 48 for controlling the servomotor 48. The servo driver 46 is a personal computer (PC) or a microcomputer including a central processing unit (CPU), a main memory, and a storage apparatus, etc. The storage apparatus stores software such as programs for realizing the functions of the servo driver 46. When the CPU operates according to the software stored in the storage apparatus, the servo driver 46 functions as specific means achieved by a collaboration between software and hardware resources. Alternatively, the servo driver 46 may include a plurality of modules that integrally realize its functions.

The spindle 50 is in the form of an externally threaded rod threaded through a nut 52 mounted on a rear surface of a vertically movable plate 54. When the servo driver 46 issues a command to the servomotor 48 to energize the servomotor 48, the spindle 50 rotates about its own central axis, causing the nut 52 to move the vertically movable plate 54 vertically. The first grinding unit 10 a is fixedly mounted on a front surface of the vertically movable plate 54. The first grinding unit 10 a includes a spindle 14 a extending in the vertical directions and a spindle motor 12 a coupled to the upper end of the spindle 14 a. A wheel mount 16 a that is in the form of a circular plate is disposed on the lower end of the spindle 14 a. The wheel mount 16 a has a lower surface on which a disk-shaped grinding wheel 18 a is mounted. A plurality of grinding stones 20 a arranged in an annular array are fixed to a lower surface of the grinding wheel 18 a. When the spindle motor 12 a is energized, it rotates the grinding wheel 18 a circumferentially about a wheel rotation axis 12 c perpendicular to the lower surface of the grinding wheel 18 a.

For grinding a workpiece 1 on the chuck table 8 in the rough grinding area 6 b, the grinding wheel 18 a is rotated about the wheel rotation axis 12 c to move the grinding stones 20 a along an annular track, and the chuck table 8 is rotated about the table rotation axis 8 c. Then, the grinding feed unit 24 a is actuated to lower the first grinding unit 10 a to bring the grinding stones 20 a into contact with the reverse side 1 b of the workpiece 1 held on the chuck table 8, thereby roughly grinding the reverse side 1 b of the workpiece 1. The grinding stones 20 a are made of a binder mixed with abrasive grains of diamond or the like dispersed therein, for example. The surfaces of the grinding stones 20 a that are to be held in contact with the workpiece 1 have some abrasive grains exposed from the binder. The workpiece 1 is ground by the grinding stones 20 a when the exposed abrasive grains thereof contact the workpiece 1.

As illustrated in FIG. 1, another column 22 b is erected adjacent to the column 22 a on the rear portion of the base 4. The column 22 b supports on a front face thereof a grinding feed unit 24 b that supports the second grinding unit 10 b for movement in vertical directions. The grinding feed unit 24 b has a configuration similar to the grinding feed unit 24 a. As with the first grinding unit 10 a, the second grinding unit 10 b includes a spindle 14 b extending in the vertical directions and a spindle motor 12 b coupled to the upper end of the spindle 14 b. A wheel mount 16 b that is in the form of a circular plate is disposed on the lower end of the spindle 14 b. The wheel mount 16 b has a lower surface on which a disk-shaped grinding wheel 18 b is mounted. A plurality of grinding stones 20 b arranged in an annular array are fixed to a lower surface of the grinding wheel 18 b. When the spindle motor 12 b is energized, it rotates the grinding wheel 18 b circumferentially about a wheel rotation axis perpendicular to the lower surface of the grinding wheel 18 b.

For grinding a workpiece 1 on the chuck table 8 in the finish grinding area 6 c, the grinding wheel 18 b is rotated about the wheel rotation axis to move the grinding stones 20 b along an annular track, and the chuck table 8 is rotated about the table rotation axis 8 c. Then, the grinding feed unit 24 b is actuated to lower the second grinding unit 10 b to bring the grinding stones 20 b into contact with the reverse side 1 b of the workpiece 1 held on the chuck table 8, thereby finishingly grinding the reverse side 1 b of the workpiece 1.

For grinding a workpiece 1 using the first grinding unit 10 a, the grinding feed unit 24 a performs grinding feed at a relatively high speed to roughly grind the workpiece 1. According to the grinding feed performed by the grinding feed unit 24 a, most of a layer of the workpiece 1 to be removed by being ground by the grinding apparatus 2 is efficiently removed. For grinding the workpiece 1 using the second grinding unit 10 b, the grinding feed unit 24 b performs grinding feed at a relatively low speed to finishingly grind the workpiece 1. According to the grinding feed performed by the grinding feed unit 24 b, the workpiece 1 is ground highly accurately to a finished thickness that has been projected.

The grinding feed units 24 a and 24 b each including the servomotor 48, the spindle 50, the nut 52, etc. have been described as vertically moving mechanisms for vertically moving the grinding units 10 a and 10 b. However, the present invention is not limited to such details. The grinding feed units 24 a and 24 b may vertically move the chuck tables 8 instead of the grinding units 10 a and 10 b. In other words, the grinding feed units 24 a and 24 b have a function to relatively move the chuck tables 8 and the spindles 14 a and 14 b toward and away from each other.

A first thickness measuring unit 40 for measuring the thickness of the workpiece 1 that is being roughly ground by the first grinding unit 10 a is disposed on an upper surface of the base 4 near the rough grinding area 6 b. The first thickness measuring unit 40 may be a contact-type thickness measuring unit, for example. However, the first thickness measuring unit 40 is not limited to a contact-type thickness measuring unit but may be any of other types of thickness measuring instruments. As illustrated in FIGS. 2 and 3, the first thickness measuring unit 40 includes a first probe 40 a for measuring the height of the reverse side 1 b of the workpiece 1 by contacting the reverse side 1 b and a second probe 40 b for measuring the height of the holding surface 8 a of the chuck table 8 by contacting the holding surface 8 a. The first thickness measuring unit 40 also includes a post 40 c supporting the first probe 40 a and the second probe 40 b thereon. The first thickness measuring unit 40 can calculate the thickness of the workpiece 1 from the difference between the height of the reverse side 1 b of the workpiece 1 and the height of the holding surface 8 a of the chuck table 8.

A second thickness measuring unit 42, which is of the non-contact type, for measuring the thickness of the workpiece 1 that is being finishingly ground by the second grinding unit 10 b is disposed on an upper surface of the base 4 near the finish grinding area 6 c. The second thickness measuring unit 42 is a laser thickness measuring unit, i.e., a spectral interference laser displacement meter, for measuring the height of the reverse side 1 b of the workpiece 1 by applying a laser beam to the reverse side 1 b and detecting the laser beam reflected therefrom. However, the second thickness measuring unit 42 is not limited to such a laser thickness measuring unit.

The manner in which the grinding stones 20 a and 20 b and a workpiece 1 contact each other when the workpiece 1 is ground thereby will be described in detail below. As illustrated in FIG. 3, the holding surface 8 a of the chuck table 8 is of a conical shape whose gradient is extremely small, and the annular track along which the grinding stones 20 a and 20 b move lies over the center of the holding surface 8 a of the chuck table 8. For grinding the workpiece 1 with the grinding stones 20 a and 20 b, the grinding stones 20 a and 20 b contact the workpiece 1 in an arcuate area extending from the center of the holding surface 8 a to outer circumferential edges thereof and underlying the annular track. When the workpiece 1 is ground by the grinding stones 20 a and 20 b, the grinding wheels 18 a and 18 b rotate about the wheel rotation axis 12 c and the chuck table 8 rotates about the table rotation axis 8 c that intersects with the holding surface 8 a of the chuck table 8. Therefore, local regions of the reverse side 1 b of the workpiece 1 successively enter the arcuate area where the grinding stones 20 a and 20 b contact the workpiece 1, so that the reverse side 1 b of the workpiece 1 is ground in its entirety.

The grinding apparatus 2 includes a control unit 44 for controlling the components of the grinding apparatus 2. Specifically, the control unit 44 controls the turntable 6, the chuck tables 8, the grinding feed units 24 a and 24 b, the grinding units 10 a and 10 b, the servo driver 46, the thickness measuring units 40 and 42, etc. Furthermore, the control unit 44 also controls the transport robot 30, the positioning table 32, the loading arm 34, the unloading arm 36, the spinner cleaning apparatus 38, etc. The control unit 44 is in the form of a computer that includes a processor such as a CPU, a storage apparatus such as a flash memory, etc. When the processor operates according to software such as programs stored in the storage apparatus, the control unit 44 functions as specific means achieved by a collaboration between software and the processor (hardware resource). For grinding a workpiece 1 on the grinding apparatus 2, the control unit 44 rotates the chuck tables 8 and the grinding units 10 a and 10 b. Heretofore, it has been customary for the control unit 44 to issue a command to the servo driver 46 to energize the servomotor 48, starting to move the chuck tables 8 and the spindles 14 a and 14 b of the grinding units 10 a and 10 b toward each other.

Heretofore, predetermined target thicknesses for the workpiece 1 are registered in advance in the control unit 44. The control unit 44 controls the thickness measuring units 40 and 42 to measure the thickness of the workpiece 1 being ground. When the thickness of the workpiece 1 has reached the predetermined target thicknesses, the control unit 44 issues a command to the servo driver 46 to de-energize the servomotor 48, finishing the grinding of the workpiece 1. The control unit 44 controls the various units and measuring instruments of the grinding apparatus 2 and processes various pieces of information constantly, as described above. Consequently, after the grinding apparatus 2 has started grinding the workpiece 1, the control unit 44 may undergo a malfunction due to an excessive processing load thereon, for example. The malfunctioning control unit 44 is unable to issue a command to the servo driver 46 to finish the relative movement of the spindles 14 a and 14 b and the chuck tables 8 when the workpiece 1 is thinned to the predetermined target thicknesses. If the relative movement of the spindles 14 a and 14 b and the chuck tables 8 is not finished when it should be finished, the workpiece 1 may not only be thinned beyond the target thicknesses but also may be ground away, allowing the grinding stones 20 a and 20 b to hit the chuck tables 8 and possibly resulting in serious damage to the grinding apparatus 2.

In the grinding apparatus 2 that carries out the method of processing a workpiece according to the present embodiment, the servo driver 46 stops grinding feed on the spindles 14 a and 14 b when grinding feed has progressed to the extent that the workpiece 1 is assumed to have reached target thicknesses. In this case, even if the control unit 44 is unable to issue a command to the servo driver 46 to de-energize the servomotor 48, since the servo driver 46 de-energizes the servomotor 48, the workpiece 1 is not excessively ground.

Details of such a step will be described below. The control unit 44 calculates projected depths of cut by subtracting the predetermined target thicknesses for the workpiece 1 that have been registered in the control unit 44 in advance from the thicknesses of the workpiece 1. The control unit 44 gives a command representing the calculated projected depths of cut to the servo driver 46 for the grinding feed units 24 a and 24 b. The thicknesses of the workpiece 1 may be registered in the control unit 44 in advance or may be measured by the thickness measuring units 40 and 42 before the workpiece 1 is ground.

When the servo driver 46 is supplied with the command representing the projected depths of cut from the control unit 44, the servo driver 46 starts to move the chuck tables 8 and the spindles 14 a and 14 b toward each other. When the chuck tables 8 and the spindles 14 a and 14 b have relatively moved by target distances corresponding to the projected depths of cut, the servo driver 46 finishes the relative movement of the chuck tables 8 and the spindles 14 a and 14 b. In this case, even if the control unit 44 malfunctions and is unable to issue a command to the servo driver 46, the servo driver 46 can control the servomotor 48. In other words, the servo driver 46 stops grinding feed at the time the workpiece 1 has reached the target thicknesses. At this time, the grinding stones 20 a and 20 b may be worn to a state that is not sufficiently reflected in the target distances. However, at least the workpiece 1 is not excessively ground, and hence, the grinding apparatus 2 is prevented from being damaged.

The control unit 44 may have a function to cause the thickness measuring units 40 and 42 to repeatedly measure the thicknesses of the workpiece 1 until the thicknesses of the workpiece 1 reach the target thicknesses for the workpiece 1. The control unit 44 with such a function may calculate projected depths of cut by subtracting the target thicknesses from the measured thicknesses of the workpiece 1 and may repeatedly issue a command representing the calculated projected depths of cut to the servo driver 46. Each time the servo driver 46 is supplied with a command representing the projected depths of cut, the servo driver 46 may update the target distances based on the supplied command representing the projected depths of cut. In this case, the thicknesses of the workpiece 1 being ground are repeatedly monitored, and the target distances are corrected depending on the actual thicknesses of the workpiece 1. Since the state to which the grinding stones 20 a and 20 b are worn is reflected in the target distances, when the workpiece 1 has been ground by the grinding units 10 a and 10 b, the thicknesses of the workpiece 1 are more accurately close to the target thicknesses.

The method of processing a workpiece according to the present embodiment, which is carried out by the grinding apparatus 2, will be described below. According to the processing method, the workpiece 1 is thinned by being ground by the grinding apparatus 2. FIG. 4A is a flowchart of a sequence of steps of the method of processing a workpiece according to the embodiment. The processing method includes holding step S1 of holding the workpiece 1 on the holding surface 8 a of the chuck table 8, depth-of-cut command issuing step S2 of repeatedly issuing a command representing a projected depth of cut to the servo driver 46, and grinding step S3 of grinding the workpiece 1. The steps will be described in detail below.

In holding step S1, as illustrated in FIG. 1, for example, the workpiece 1 with the protective member 3 affixed to the face side 1 a thereof is housed in the cassette 28 a, and the cassette 28 a is introduced into the grinding apparatus 2. Then, using the transport robot 30, for example, the workpiece 1 housed in the cassette 28 a is unloaded therefrom and transported to the positioning table 32. After the workpiece 1 has been positioned in a predetermined position by the positioning table 32, the workpiece 1 is transported onto the chuck table 8 positioned in the loading/unloading area 6 a by the loading arm 34. The face side 1 a of the workpiece 1 faces downwardly such that the reverse side 1 b thereof which is to be ground faces upwardly. The workpiece 1 is placed on the holding surface 8 a of the chuck table 8, and the suction source connected to the chuck table 8 is actuated to hold the workpiece 1 under suction on the chuck table 8.

According to the processing method, depth-of-cut command issuing step S2 and grinding step S3 are then carried out. FIG. 4B is a flowchart of a sequence of steps of depth-of-cut command issuing step S2, and FIG. 4C is a flowchart of a sequence of steps of grinding step S3. FIG. 2 is a schematic perspective view illustrating depth-of-cut command issuing step S2 and grinding step S3. FIG. 3 is a schematic cross-sectional view illustrating depth-of-cut command issuing step S2 and grinding step S3. In grinding step S3, the workpiece 1 is roughly ground in the rough grinding area 6 b by the first grinding unit 10 a. In grinding step S3, furthermore, the workpiece 1 is finishingly ground in the finish grinding area 6 c by the second grinding unit 10 b. The step of roughly grinding the workpiece 1 in the rough grinding area 6 b by the first grinding unit 10 a will be described by way of example below. The step of finishingly grinding the workpiece 1 in the finish grinding area 6 c by the second grinding unit 10 b is similarly carried out and its description will be omitted below. Grinding step S3 is started while depth-of-cut command issuing step S2 is being in progress and is subsequently progressed together with depth-of-cut command issuing step S2. Depth-of-cut command issuing step S2 and grinding step S3 will be individually described below.

First, depth-of-cut command issuing step S2 will be described below. The control unit 44 has registered therein the target thickness to be achieved for the workpiece 1 when the rough grinding is completed. In depth-of-cut command issuing step S2, the first thickness measuring unit 40 measures the thickness of the workpiece 1 in step S21 (see FIG. 4B). The control unit 44 compares the thickness of the workpiece 1 and the target thickness with each other and determines whether or not the thickness of the workpiece 1 has reached the target thickness in step S22. Normally, immediately after depth-of-cut command issuing step S2 has started, the thickness of the workpiece 1 has not yet reached the target thickness. If the thickness of the workpiece 1 has not reached the target thickness, then the control unit 44 calculates a projected depth of cut by subtracting the target thickness from the measured thickness of the workpiece 1 in step S23. Thereafter, the control unit 44 issues a command representing the calculated projected depth of cut to the servo driver 46 of the grinding feed unit 24 a in step S24.

When the control unit 44 issues the command representing the calculated projected depth of cut to the servo driver 46, grinding step S3 starts, as described later, to begin grinding and thinning the workpiece 1. In depth-of-cut command issuing step S2, while the workpiece 1 is being thinned, the first thickness measuring unit 40 periodically measures the thickness of the workpiece 1 in step S21 and the control unit 44 determines again whether or not the thickness of the workpiece 1 has reached the target thickness in step S22.

If the control unit 44 decides that the thickness of the workpiece 1 has not reached the target thickness, then the control unit 44 calculates a new projected depth of cut by subtracting the target thickness from the re-measured thickness of the workpiece 1 in step S23. Normally, while the grinding step is in progress, the workpiece 1 is progressively thinned, and the projected depth of cut is reduced by as much as the workpiece 1 is thinned. The control unit 44 issues a command representing the newly calculated projected depth of cut to the servo driver 46 in step S24. On the other hand, if the control unit 44 decides that the thickness of the workpiece 1 has reached the target thickness due to the progress of the grinding step in step S22, the control unit 44 does not issue a new command to the servo driver 46. Then, depth-of-cut command issuing step S2 comes to an end.

To sum up, in depth-of-cut command issuing step S2, the first thickness measuring unit 40 repeatedly measures the thickness of the workpiece 1 until the thickness of the workpiece 1 reaches the target thickness registered beforehand in the control unit 44 for the workpiece 1. The control unit 44 calculates a projected depth of cut by subtracting the target thickness from the measured thickness of the workpiece 1 and repeatedly issues a command representing the calculated projected depth of cut to the servo driver 46 of the grinding feed unit 24 a.

Next, grinding step S3 will be described below. Grinding step S3 starts while depth-of-cut command issuing step S2 is in progress. In grinding step S3, the chuck table 8 is rotated about the table rotation axis 8 c while at the same time the grinding wheel 18 a is rotated about the wheel rotation axis 12 c. When the servo driver 46 receives a command representing the projected depth of cut from the control unit 44 in step S31 (see FIG. 4C), the servomotor 48 of the grinding feed unit 24 a starts relatively moving the chuck table 8 and the spindle 14 a toward each other in step S32. For example, the servo driver 46 starts energizing the servomotor 48 to lower the spindle 14 a of the first grinding unit 10 a at a predetermined speed.

The servo driver 46 controls the servomotor 48 under feedback control while monitoring the operation of the servomotor 48, so that the servomotor 48 can operate to achieve a predetermined task. When the grinding stones 20a moving along the annular track contact the reverse side 1 b of the workpiece 1 held on the holding surface 8 a of the chuck table 8, the workpiece 1 starts to be ground by the grinding stones 20 a, beginning to be thinned. The servo driver 46 controls the servomotor 48 in order to achieve a state in which the chuck table 8 and the spindle 14 a of the first grinding unit 10 a are close to each other by a target distance corresponding to the projected depth of cut. The servo driver 46 then determines whether or not the chuck table 8 and the spindle 14 a have relatively moved by the target distance in step S33. Normally, immediately after grinding step S3 has started, the distance that the chuck table 8 and the spindle 14 a have relatively moved does not reach the target distance.

If it is decided that the chuck table 8 and the spindle 14 a have not relatively moved by the target distance, then it is determined whether or not the target distance needs to be updated. Specifically, if a command representing a new projected depth of cut has been supplied from the control unit 44 to the servo driver 46 as a result of the progress of depth-of-cut command issuing step S2 in step S34, then the servo driver 46 updates the target distance on the basis of the new projected depth of cut in step S35. On the other hand, if a command representing a new projected depth of cut has not been supplied from the control unit 44 to the servo driver 46 in step S34, then the servo driver 46 does not update the target distance, and grinding step S3 continues to be carried out on the basis of the latest target distance registered in the servo driver 46 thus far.

In grinding step S3, regardless whether or not the target distance has been updated, it is repeatedly determined periodically whether or not the distance that the chuck table 8 and the spindle 14 a have relatively moved has reached the target distance in step S33. If the distance that the chuck table 8 and the spindle 14 a have relatively moved has not reached the target distance and if the control unit 44 issues a command representing a new projected depth of cut to the servo driver 46 in step S34, then the servo driver 46 updates the target distance on the basis of the new projected depth of cut in step S35. In other words, each time the servo driver 46 is supplied with a command representing a newly calculated depth of cut from the control unit 44 until the chuck table 8 and the spindle 14 a have relatively moved by the target distance, the servo driver 46 updates the target distance on the basis of the newly calculated depth of cut.

If the distance that the chuck table 8 and the spindle 14 a have relatively moved has reached the target distance in step S33, the servo driver 46 de-energizes the servomotor 48, finishing the relative movement of the chuck table 8 and the spindle 14 a in step S36. In the method of processing a workpiece according the present embodiment, depth-of-cut command issuing step S2 and grinding step S3 are carried out to grind the workpiece 1 held on the chuck table 8 by the projected depth of cut to thin the workpiece 1 to the target thickness.

For example, when the grinding of the workpiece 1 is in progress, the rate at which the thickness of the workpiece 1 is reduced may become small compared with the distance by which the chuck table 8 and the spindle 14 a are relatively moved, due to wear on the grinding stones 20 a or the like. In this case, unless the target distance is updated, the thickness of the workpiece 1 upon completion of grinding step S3 may not necessarily reach the target thickness registered in the control unit 44. In the method of processing a workpiece according to the present embodiment, the thickness of the workpiece 1 is repeatedly measured, and a depth of cut required for the workpiece 1 to reach the target thickness is continuously calculated as a projected depth of cut. At this time, since the progress of the grinding and wear on the grinding stones 20 a are reflected in the projected depth of cut, the workpiece 1 is ground by the grinding stones 20 a until the thickness thereof reaches the target thickness.

In the method of processing a workpiece according to the present embodiment described above, even in the event of a malfunction of the control unit 44, the workpiece 1 is ground generally to the target thickness without causing damage to the grinding apparatus 2. One of the reasons for this is that depth-of-cut command issuing step S2 is realized mainly by the function of the control unit 44 whereas grinding step S3 is realized mainly by the function of the servo driver 46. Next, a case in which the control unit 44 malfunctions while the method of processing a workpiece according to the present embodiment is being carried out will be described below.

A malfunction of the control unit 44 refers to a condition in which the control unit 44 stops functioning due to an abnormal situation and is unable to process information and issue commands. Heretofore, the control unit 44 controls the servo driver 46 to de-energize the servomotor 48 when the thickness of the workpiece 1 has reached the target thickness. In a case in which the control unit 44 malfunctions, the control unit 44 is unable to issue a command for de-energizing the servomotor 48 to the servo driver 46.

In the method of processing a workpiece according to the present embodiment, in a case in which the control unit 44 malfunctions while depth-of-cut command issuing step S2 and grinding step S3 are in progress after holding step S1 has been performed, the control unit 44 is unable to continue depth-of-cut command issuing step S2. Specifically, the malfunctioning control unit 44 is unable to measure the thickness of the workpiece 1 using the first thickness measuring unit 40 in step S21 and is unable to determine whether or not the thickness of the workpiece 1 has reached the target thickness in step S22. The malfunctioning control unit 44 is unable to calculate a projected depth of cut in step S23 and is unable to issue a command representing a projected depth of cut to the servo driver 46 in step S24.

In grinding step S3, a command representing a new projected depth of cut is not supplied from the control unit 44 in step S34, and the target distance is not updated in step S35. In other words, in the case of a malfunction of the control unit 44, it is repeatedly determined in grinding step S3 whether the chuck table 8 and the spindle 14 a have relatively moved by the target distance in step S33. When the chuck table 8 and the spindle 14 a have relatively moved by the target distance, the servo driver 46 stops relatively moving the chuck table 8 and the spindle 14 a in step S36. In the method of processing a workpiece according to the present embodiment, therefore, grinding step S3 is normally finished even if the control unit 44 malfunctions.

In this case, since the target distance is not updated, the thickness of the ground workpiece 1 may not possibly reach the target thickness due to wear on the grinding stones 20 a, etc. However, any resultant loss is much smaller than if the workpiece 1 is excessively ground as is the case with the conventional practice because the grinding apparatus 2 suffers no physical damage and the workpiece 1 can be ground again to the target thickness after the control unit 44 has been recovered from the malfunction. Needless to say, it may be not necessary to grind the workpiece 1 again.

In the method of processing a workpiece according to the present embodiment, as described above, depth-of-cut command issuing step S2 may be finished for the reason that the control unit 44 malfunctions. The processing method according to the present embodiment is advantageous in that the workpiece 1 will not be lost in a case in which the control unit 44 malfunctions and there are benefits available in a case in which the control unit 44 does not malfunction. In the processing method according to the present embodiment, for example, even in the event of a malfunction of the control unit 44, the grinding apparatus 2 is prevented from suffering damage which would otherwise be caused if the chuck table 8 were ground by the grinding stones 20 a and 20 b, for example. This means that after the control unit 44 has malfunctioned, the time required for the grinding apparatus 2 to recover from the malfunction is shortened and the cost required for the grinding apparatus 2 to recover from the malfunction is reduced.

Consequently, since the grinding apparatus 2 has reduced downtime, the grinding apparatus 2 has an increased processing efficiency for successively processing a plurality of workpieces 1, and the number of workpieces 1 that can be processed per unit time is increased. As no workpieces 1 are lost by excessive grinding and no loss needs to be made up for, the processing cost is reduced. In other words, the processing efficiency for processing workpieces 1 while the control unit 44 is not malfunctioning is increased. Specifically, the processing method according to the present embodiment gives rise to a situation in which the workpiece 1 is not lost and the grinding apparatus 2 is not damaged even if the control unit 44 malfunctions, and makes it meaningful to process the workpiece 1 in such a situation. Therefore, the processing method according to the present embodiment does not require the control unit 44 to malfunction and is not limited to a case in which the control unit 44 malfunctions.

As described above, in the processing method according to the present embodiment, even if the control unit 44 of the grinding apparatus 2 malfunctions while the workpiece 1 is being ground, the workpiece 1 is not likely to be excessively ground and no damage is inflicted on the grinding apparatus 2.

The present invention is not limited to the above details of the present embodiment, and various changes and modifications may be made therein. According to the above embodiment, for example, the manner in which the workpiece 1 is ground in the rough grinding area 6 b by the first grinding unit 10 a has been described in detail above. However, the present invention is not limited to the grinding of the workpiece 1 in the rough grinding area 6 b by the first grinding unit 10 a and is applicable to the grinding of the workpiece 1 in the finish grinding area 6 c by the second grinding unit 10 b. The grinding apparatus 2 with the two grinding units, i.e., the first grinding unit 10 a and the second grinding unit 10 b, has been illustrated above. However, the present invention is not limited to a grinding apparatus with two grinding units. The method of processing a workpiece according to an aspect of the present invention may be carried out by a grinding apparatus with a single grinding unit or a grinding apparatus with three or more grinding units.

Furthermore, in the above embodiment, malfunctioning of the control unit 44 of the grinding apparatus 2 and operation of the servo driver 46 have been described above by way of example. The present invention is also advantageous in a case in which the grinding apparatus 2 does not operate normally for other reasons. For example, the invention is also advantageous in a case in which the thickness measuring units 40 and 42 malfunction and are unable to measure the thickness of the workpiece 1 while the workpiece 1 is being ground and in a case in which signal wires as a path for transmitting commands from the control unit 44 to the servo driver 46 are broken.

Moreover, in the above embodiment, the case in which the thickness of the workpiece 1 is repeatedly measured, the projected depth of cut is updated, and the command representing the updated projected depth of cut is sent from the control unit 44 to the servo driver 46 in depth-of-cut command issuing step S2 has been described above. In addition, the case in which the servo driver 46 updates the target distance each time it is supplied with the command representing the updated projected depth of cut from the control unit 44 in grinding step S3 has been described above. However, the present invention is not limited to such details. According to an aspect of the present invention, the thickness of the workpiece 1 may not be repeatedly measured, and the projected depth of cut and the target distance may not be updated. Specifically, a projected depth of cut may be calculated by subtracting the target thickness for the workpiece 1 from the thickness of the workpiece 1 prior to being ground, and thereafter the chuck table 8 and the spindles 14 a and 14 b may be relatively moved by a target distance corresponding to the calculated projected depth of cut, whereupon the step of grinding the workpiece 1 may be finished.

Similarly, in this case, since the grinding of the workpiece 1 may be finished without the control unit 44 issuing a command for stopping the relative movement of the chuck table 8 and the spindles 14 a and 14 b, problems such as losing the workpiece 1 in the event of a malfunction of the control unit 44 do not arise. In addition, as the thickness of the workpiece 1 being ground by the grinding apparatus 2 does not need to be measured, the thickness measuring units 40 and 42 may be simplified or omitted.

The present invention is not limited to the details of the above described preferred embodiment. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention. 

What is claimed is:
 1. A method of processing a workpiece by grinding the workpiece on a grinding apparatus including a chuck table having a holding surface for holding the workpiece thereon, the chuck table being rotatable about an axis transverse to the holding surface, a spindle for circumferentially rotating a disk-shaped grinding wheel mounted thereon and including grinding stones, a grinding feed unit having a servomotor for relatively moving the chuck table and the spindle toward and away from each other, a servo driver for sending signals to the servomotor of the grinding feed unit, and a control unit for controlling the servo driver, the method comprising: a holding step of holding the workpiece on the holding surface of the chuck table; a depth-of-cut command issuing step of calculating a projected depth of cut by subtracting a target thickness registered in advance in the control unit for the workpiece from a thickness of the workpiece and issuing a command representing the calculated projected depth of cut from the control unit to the servo driver of the grinding feed unit; and a grinding step of relatively moving the chuck table and the spindle toward each other with the servomotor of the grinding feed unit while the chuck table and the grinding wheel are being rotated about respective axes thereof, thereby grinding the workpiece held on the chuck table with the grinding stones, wherein in the grinding step, relative movement of the chuck table and the spindle is finished by the servo driver when the chuck table and the spindle have relatively moved by a target distance corresponding to the projected depth of cut.
 2. The method of processing a workpiece according to claim 1, wherein the grinding apparatus further includes a thickness measuring unit for measuring the thickness of the workpiece held on the holding surface of the chuck table, the depth-of-cut command issuing step includes repeatedly measuring the thickness of the workpiece with the thickness measuring unit until the thickness of the workpiece reaches the target thickness, updating the projected depth of cut with a value calculated by subtracting the target thickness from the measured thickness of the workpiece, and repeatedly issuing a command representing the updated projected depth of cut from the control unit to the servo driver of the grinding feed unit, and the grinding step includes updating the target distance on the basis of the updated projected depth of cut by the servo driver each time the servo driver is supplied with the command representing the updated projected depth of cut from the control unit until the chuck table and the spindle have relatively moved by the target distance. 